96 



BALANOGLOSSIDA. 



kowalevskii are known. The eggs of this species are laid singly 

 and deposited in the muddy sand which the animal inhabits. 

 Segmentation is regular and complete and leads to the forma- 

 tion of a spherical blastosphere which becomes a gastrula by 

 invagination (Fig. 71, A). The blastopore narrows and a 

 circle of cilia is formed round it ; it eventually closes up at that 

 pole of the egg which will become, as shown by the persistent 

 ciliary circle, the future hind end ; i.e. it closes in the posi- 

 tion of the future anus. The embryo becomes covered with fine 

 cilia and a tuft of longer cilia is formed at the anterior end. The 



embryo elongates and 

 two transversely directed 

 grooves appear, marking 

 out the three regions ; 

 proboscis, collar, and 

 trunk (Fig. 71, B). The 

 embryo hatches at about 

 this stage and lives as a 

 free larva which at first 

 swims in the mud by 

 means of its cilia, and 

 later burrows in the mud 

 by means of its proboscis 

 (Fig. 71, C). A groove 

 appears in the collar re- 

 gion in the dorsal middle 

 line marking the position 

 of the medullary nerve- 

 cord which is now being developed. This groove is a temporary 

 structure and does not appear to participate in the formation 

 of the collar nerve-cord which arises by delamination from the 

 ectoderm. The first gill-slits now appear just behind the collar 

 as a pair of perforations, and the mouth is formed as a minute 

 perforation in the groove between the collar and proboscis. 

 The anus is formed rather later at the hind end. 



Meanwhile the five coelomic pouches have made their appear- 

 ance ; one unpaired pouch in front and two pairs behind (Fig. 

 72). These become closed from the gut and persist as the 

 coelomic sacs. All the mesodermal tissues are derived from 

 the walls of these sacs. 



FIG. 71. Free-swimming larvae of Dolichoglossus 

 kowalevskii in different stages of development 

 (after Bateson, from Korschelt and Heider). e 

 . proboscis ; k branchial pore ; kr collar. 



